A2 Principi di Astrofisica. Coordinate Celesti

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1 A2 Principi di Astrofisica Coordinate Celesti

2 ESO La Silla Tel. 3.6m

3 Celestial Sphere Our lack of depth perception when we look into space creates the illusion that Earth is surrounded by a celestial sphere. In reality, stars that appear very close together in our sky may actually lie at very different distances from Earth. Remember that we are on the INSIDE of the sphere (on Earth) looking out!

4 Points on the Celestial Sphere North and south celestial poles Celestial equator REMEMBER: These are points /lines on the celestial sphere and NOT on the Earth From now on: equator = celestial equator

5 The Dome of the Local Sky Zenith Nadir Horizon Meridian Transit

6 Horizon coordinate system - coordinates are measured with respect to horizon - change with time and depend on observer Azimuth: 0 to 360 degrees around horizon from north towards east 0 = North, 90 = East, 180 = South, 270 = West Altitude: 0 to 90 degrees up from horizon 0 = Horizon, 90 = Zenith

7 Ecliptic Plane Plane containing the Sun and planets Ecliptic is tilted 23.5 with respect to the Equator Eclipses can only occur when the moon crosses this plane

8 Ecliptic: The Sun's apparent annual path among the constellations Zodiac Constellations The constellations on the celestial sphere through which the ecliptic passes Origin of Astrology (Zodiac Sign)

9 Cardinal Points on the Ecliptic Vernal Equinox Sun rises due East and sets due West Length of day = length of night = 12 hours Summer Solstice Sun is highest in the sky (this is why it s so hot during summer) Autumnal Equinox Winter Solstice Sun is lowest in the sky (this is why it s so cold during winter)

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11 Equatorial coordinate system - coordinates fixed on the celestial sphere - time and observer independent ( dec ) declination Analogous to latitude, but on the celestial sphere; it is the angular north-south distance between the celestial equator and a location on the celestial sphere. Measured in degrees:» 0 to 90 north from celestial equator» 0 to -90 south from celestial equator ( RA ) right ascension Analogous to longitude, but on the celestial sphere; it is the angular east-west distance between the vernal equinox and a location on the celestial sphere. Measured in units of time: hours, minutes, seconds» 0 h 24 h from Vernal Equinox towards east» Ex. Sirius has RA = 6 h 45 m OR 6:45 Don t confuse RA with time on your watch!

12 Equatorial coordinate system Comparing latitude and longitude to declination and right ascension

13 RA and Dec of the Cardinal Points on the Ecliptic Vernal Equinox Sun appears on March 21 RA = 0h Dec = 0 Summer Solstice Sun appears on June 21 RA = 6h Dec = 23.5 Autumnal Equinox Sun appears on Sept. 21 RA = 12h Dec = 0 Winter Solstice Sun appears on Dec. 21 RA = 18h Dec = -23.5

14 RA and Dec of the Cardinal Points on the Ecliptic Declination 23.5 Equator 0 h 6 h 12 h 18 h 24 h Ecliptic Vernal Equinox Sun appears on March 21 RA = 0h Dec = 0 Summer Solstice Sun appears on June 21 RA = 6h Dec = 23.5 Autumnal Equinox Sun appears on Sept. 21 RA = 12h Dec = 0 Winter Solstice Sun appears on Dec. 21 RA = 18h Dec = -23.5

15 Example: where is Vega? Its declination tells us that it is north of the celestial equator. We can interpret its right ascension in two ways: As an angle, it means Vega is about 279 east of the vernal equinox As a time, it means Vega crosses the meridian about 18 hours 35 minutes after the spring equinox.

16 Understanding Local Skies 3 classes of stars: circumpolar north - always visible circumpolar south - never visible rising and setting

17 Understanding Local Skies The sky at the North Pole.

18 Understanding Local Skies The sky at the equator

19 Understanding Local Skies The sky at 40 N latitude.

20 Understanding Local Skies The sky at 30 S latitude.

21 The altitude of the celestial pole in your sky is equal to your latitude. Everything in the sky rotates around the north celestial pole